Mobile radio systems today use various mobile radio standards such as the Global System for Mobile communication, GSM, Enhanced Data Rates for GSM Evolution, EDGE, Universal Mobile Telecommunications Standard, UMTS, or others. In this case, radio-frequency signals are used for transmission. Other systems also use radio-frequency signals for transmission.
To generate and receive the radio-frequency transmission/reception signals, use is increasingly being made of digitally controlled oscillators, DCOs. As its output signal, a DCO generates a frequency signal on the basis of a digital frequency word. In addition, a digital phase locked loop with a DCO on a semiconductor body requires less space than a corresponding phase locked loop with an analog-controlled voltage controlled oscillator, VCO.
An output signal from the DCO is supplied as an oscillator signal to a phase detector, to a frequency detector or to a phase/frequency detector in order to derive a digital error signal, describing a frequency error or phase error in the oscillator signal, in comparison with a reference signal, which represents a reference frequency. The error signal is usually processed using a digital loop filter to form a control signal for actuating the DCO.
When implementing fully digital phase locked loops, a limited resolution for the digital phase/frequency detector may result in periodic patterns in the digital error signal. Although the error signal is filtered in the phase locked loop by the loop filter, which usually has the nature of a low-pass filter, the frequency properties of the periodic patterns mean that they are rejected only inadequately. The effect may therefore arise that a low-frequency pattern also becomes visible in the output signal from the phase locked loop, and hence a prescribed spectral mask is possibly violated. By way of example, integrated phase noise in the phase locked loop may also be raised.
Such periodic patterns may arise particularly often when the frequency of the oscillator signal is close to an integer multiple of the reference frequency. It is sometimes possible to determine the frequencies at which the periodic patterns arise analytically. To reduce the occurrence of the periodic patterns, the reference frequency signal can be provided with jitter, for example. This requires additional analog circuit blocks, however.